1. Field of the Invention
The present invention relates to a scanning apparatus for scanning a document and guiding light from a document to a photosensitive body in an image forming apparatus, e.g., an electronic copying machine.
2. Description of the Related Art
An optical unit and a driver are arranged in a scanning apparatus of an electronic copying machine. The optical unit serves to illuminate a document placed on a document table and guides the light reflected by the document onto a photosensitive drum. The driver serves to move the optical unit along the document table so as to scan. The driver comprises a carriage for supporting the optical unit. Both the ends of the carriage are respectively coupled to a pair of timing belts. These timing belts are respectively looped around driving and driven timing pulleys. The driving pulleys are fixed to a single shaft. Annular groove is formed in an end portion of shaft. Each timing pulley has cylindrical portion. Female thread portion is formed in cylindrical portion. Male screw is threadably engaged with female thread portion. The leading end of male screw is brought into contact with groove of shaft. As a result, driving timing pulley is fixed to shaft. The phases of driving timing pulley and shaft in a rotational direction can be shifted from each other by loosening male screw. Timing belt is looped around driving timing pulley.
Driving timing pulley and shaft are coupled to each other only by friction between male screw and shaft. For this reason, when a large torque is applied from timing belt to driving timing pulley, a slip may be caused between driving timing pulley and shaft, thereby shifting the phases of driving timing pulley and shaft from each other. In this case, the carriage is moved while it is tilted, and hence satisfactory scanning cannot be performed.